Stock prices can go down as well as up. Never in financial history has this adage been more apt than on 6 May 2010. Then, the so-called “Flash Crash” sent shocks waves through global equity markets. The Dow Jones experienced its largest ever intraday point fall, losing $1 trillion of market value in the space of half an hour. History is full of such fat-tailed falls in stocks. Was this just another to add to the list, perhaps compressed into a smaller time window?

No. This one was different. For a time, equity prices of some of the world’s biggest companies were in freefall. They appeared to be in a race to zero. Peak to trough, Accenture shares fell by over 99%, from $40 to $0.01. At precisely the same time, shares in Sotheby’s rose three thousand-fold, from $34 to $99,999.99. These tails were not just fatter and faster. They wagged up as well as down.

The Flash Crash left market participants, regulators and academics agog. More than one year on, they remain agog. There has been no shortage of potential explanations. These are as varied as they are many: from fat fingers to fat tails; from block trades to blocked lines; from high-speed traders to low-level abuse. From this mixed bag, only one clear explanation emerges: that there is no clear explanation. To a first approximation, we remain unsure quite what caused the Flash Crash or whether it could recur.

That conclusion sits uneasily on the shoulders. Asset markets rely on accurate pricing of risk. And financial regulation relies on an accurate reading of markets. Whether trading assets or regulating exchanges, ignorance is rarely bliss. It is this uncertainty, rather than the Flash Crash itself, which makes this an issue of potential systemic importance.

In many respects, this uncertainty should come as no surprise. Driven by a potent cocktail of technology and regulation, trading in financial markets has evolved dramatically during the course of this century. Platforms for trading equities have proliferated and fragmented. And the speed limit for trading has gone through the roof. Technologists now believe the sky is the limit.

This rapidly-changing topology of trading raises some big questions for risk management. There are good reasons, theoretically and empirically, to believe that while this evolution in trading may have brought benefits such as a reduction in transaction costs, it may also have increased abnormalities in the distribution of risk and return in the financial system. Such abnormalities hallmarked the Flash Crash. This paper considers some of the evidence on these abnormalities and their impact on systemic risk.

Regulation has thin-sliced trading. And technology has thin-sliced time. Among traders, as among stocks on 6 May, there is a race to zero. Yet it is unclear that this race will have a winner. If it raises systemic risk, it is possible capital markets could be the loser. To avoid that, a redesign of mechanisms for securing capital market stability may be needed.

The trigger was indeed an algorithm, but not one of the sophisticated ultra-fast high-frequency trading programs. It was a simple ‘volume participation’ algorithm, and while the official investigation does not name the firm that deployed it, market participants seem convinced that it was the Kansas City investment managers Waddell & Reed. The firm’s goal was to protect the value of a large position in the stock market against further declines, and it did this by programming the algorithm to sell 75,000 index future contracts. (These contracts track the S&P 500 stock-market index, and each contract was equivalent to shares worth a total of around $55,000. The seller of index futures makes money if the underlying index falls; the buyer gains if it rises.) The volume participation algorithm calculated the number of index futures contracts that had been traded over the previous minute, sold 9 per cent of that volume, and kept going until the full 75,000 had been sold. The total sell order, worth around $4.1 billion, was unusually large, though not unprecedented: the SEC/CFTC investigators found two efforts in the previous year to sell the same or larger quantities of futures in a single day. But the pace of the sales on 6 May was very fast.

The Flash Crash: The Impact of High Frequency Trading on an Electronic Market

Abstract: The Flash Crash, a brief period of extreme market volatility on May 6, 2010 raised questions about the current structure of the U.S. financial markets. We use audit-trail data to describe the structure of the E-mini S&P 500 stock index futures market on May 6. We ask three questions. How did High Frequency Traders (HFTs) trade on May 6? What may have triggered the Flash Crash? What role did HFTs play in the Flash Crash? We conclude that HFTs did not trigger the Flash Crash, but their responses to the unusually large selling pressure on that day exacerbated market volatility.